1. Field of the Invention
The present invention generally relates to a method and apparatus for controlling a motor.
2. Description of the Related Art
In work-purpose vehicles such as a battery vehicle, a motor is driven by a battery which can flow large current in a low voltage. The motor is controlled through a vector control method using a torque command current Iq* and an exciting command current Id*. Normally, the torque command current Iq* and the exciting command current Id* are calculated based upon a torque command T* by use of basic equations which indicates characteristics of the motor. However, since the basic characteristics of a magnetic flux is not linear in the low-voltage/large-current motor, a torque command current Iq* and an exciting command current Id* which are obtained from the basic equations are different from a torque current Id and an exciting current Id in which the motor can be operated in the highest efficiency.
Accordingly, a method of determining a torque command currents Iq* and an exciting command currents Id* at the time when the motor can be effectively driven is demanded.
In conjunction with the above description, a method is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 6-296387), in which a lookup table is referred to convert a torque command value T* supplied from a host into an output torque estimation T*′. This conventional example relates to a torque control method of an induction motor under a torque control, and a command current of the induction motor is inputted as a primary current input to a T-type equivalent circuit. An exciting current and a secondary current are calculated from a rotation frequency and a slip frequency of the induction motor to calculate the output torque estimation. A relationship between the host torque command and the output torque estimation is previously determined to each of a plurality of rotation frequencies of the induction motor. Thus, when one of the plurality of relations is selected based on a rotation frequency, a host torque command is converted into an output torque estimation by use of the selected relationship and the torque control of the induction motor is carried out based upon the determined output torque estimation.